Portable applications have a limited power supply, such as a battery. Consequently, power conservation and efficiency are more important in the design of portable applications than in applications (e.g., a desktop application) that are connected to a virtually unlimited power supply (e.g., wall outlet).
From a digital designer's perspective, the simple and obvious manner to save power is to decrease the supply voltage for the digital circuits. However, for analog circuits, a reduced power supply voltage often introduces severe design constraints and challenges. For example, an analog design circuit that operates well at a particular power supply voltage may fail to operate in a desired manner or to meet design requirements when the power supply voltage is decreased.
One important circuit is a driver circuit for driving light emitting diodes (LEDs). LEDs are found in many portable applications including back lighting, displays (e.g., for cellular telephones, laptop computers, personal digital assistants (PDAs), etc.).
In a typical system that has a 5V supply voltage, a current mirror is utilized as a LED driver. FIG. 1 illustrates such an approach. The output current (I_out) provided to the LED is a mirror ratio of the input bias current (I_bias). In the ideal case, the output current (I_out) is constant while the M2 transistor operates in a saturation region. However, when the M2 transistor operates in a triode region, the output current drops rapidly with a decreasing drain to source voltage (V_ds). This is illustrated in the I_out versus V_ds graph of FIG. 2. It is noted that the output impedance of the M2 transistor can also affect the output current (I_out) when the M2 transistor is operating in the saturation region.
To minimize the effect of the output impedance on the output current (I_out), the current mirror is not designed with a minimum transistor length. Increasing the device length improves current tolerance because the output impedance increases. However, increasing the device length of the M2 transistor also increases the saturation voltage.
It is noted that since the minimum supply voltage is the saturation voltage of the M2 transistor plus the forward voltage of the LED, increasing the channel length of the transistors tends to limit the minimum supply voltage.
Based on the foregoing, there remains a need for a driver circuit for applications with low power voltage supplies that overcomes the disadvantages set forth previously.